1. Field
This application relates to an analyzer used to determine oscillation properties of a golf shaft assembly.
2. Prior Art
Golf clubs have three main components; a shaft, a club head attached to the shaft's tip end, and a grip on the shaft's butt end. The masses of these three components and how they lie along the length of the club determine the club's mass distribution. Since the club head is relatively inflexible and the grip relatively flexible compared to the shaft, the shaft is primarily responsible for the flexibility of the golf club. Shafts are normally tapered, round, hollow, and may have a wall thickness that varies along their axes, and these properties, along with the material used in the shaft and its overall length, determine the shaft's flexibility profile. The club's mass distribution and flexibility profile determine the club's oscillation properties. Two of these oscillation properties which are important to club fitters are the oscillation's fundamental natural frequency and a preferred rotational orientation of the oscillation around the shaft's axis which minimizes wobbling during a golf swing.
The oscillation's natural frequency will be discussed first. Shaft manufacturers use a shaft flexibility rating in an attempt to predict a frequency of a finished club. A standard flex rating system uses LL, or 2.0, for ladies light, L, or 3.0, for ladies, A, or 3.0, for amateur or senior, R, or 4.0, for regular, S, or 5.0, for stiff, X, or 6.0, for extra stiff, and XX, or 8.0, for extra-extra stiff. There are problems with this rating system, one being that there is no industry standard for shaft stiffness. Another problem is that heads have different masses; a driver head is lighter than say a 3 wood head, but the same shaft may be installed in both. Mass affects frequency, so just knowing a shaft flex rating does not allow knowing the frequency of a finished golf club with a particular head. Another problem is that shafts come from the manufacturer with a length longer than normally needed and the shaft is shortened to meet the requirements of the intended golfer. Shaft length affects frequency, so knowing a flex rating for a shaft having an “as manufactured” length does not allow knowing the frequency of a finished club with a shortened shaft.
Golf shaft assembly frequency analyzers are available which use an electronic counting mechanism to determine oscillation frequency. These analyzers use a clamp to hold the upper 12.7 cm (5 inches) of the shaft's butt end, an industry standard. The shaft assembly to be tested can be a finished club, or if the shaft does not have a head installed, an industry standard is to create a shaft assembly by temporarily installing a 205 gram weight on the tip end of the shaft to simulate the weight of a head. Testing can be done with or without a grip installed. With the shaft assembly held in the clamp, the tip end of the assembly is twanged; it is deflected and released. The shaft assembly oscillates, and usually using some form of light sensor or pressure transducer, an electronic counter measures the natural frequency of oscillation, usually expressed in units of cycles per minute (CPM).
The second important property of a golf shaft assembly's oscillation is a preferred rotational orientation of the oscillation around the shaft's axis. Most shafts have a “spine” which lies along the edge of the shaft where the material used to make the shaft is joined. This spine and other manufacturing irregularities can result in a flexibility profile of the shaft which is not constant with rotation around the shaft's axis, and these irregularities can cause wobbling for many rotational orientations of the oscillation around the shaft's axis.
A shaft which is not flexed has an axis which is essentially a straight line. When a shaft is bent, its axis becomes an arc, and a term “deflection plane” can be used to define a plane which “on average” contains this arc. The term “on average” must be used because if a shaft has a flexibility profile not constant with axial rotation, the arc may be “splayed” out to one side; the axis is not contained in the deflection plane but bulges slightly to one side or the other. Most shafts normally, however, have two essentially orthogonal deflection planes in which its axis does essentially lie in a plane. Once started oscillating in one of these planes, it will oscillate with its axis essentially always in that plane, and such as plane is called a “flat line oscillation” plane, or FLO plane. Oscillation in which the shaft's axis always essentially lies in a FLO plane can be called “planar oscillation”, “pure oscillation”, or FLO.
FLO testing normally involves twanging the tip end of a shaft assembly and observing its subsequent oscillation. The deflection is done with several deflection planes located rotationally around the shaft's axis, and almost all these deflection planes will result in shaft oscillations which wobble. As discussed above, there are normally two deflection planes which will result in FLO; the shaft is observed to be in essentially pure oscillation without wobble. The tip end follows a straight line, or “flat line”, rather than somewhat of an oval shape, hence the term flat line oscillation. Once these FLO planes are found, the club head is normally installed on the shaft so that one of them is perpendicular to the face of the head. This will cause most of the shaft's flexing during the swing to be planar, minimizing undesirable wobbling during the swing.
There are some difficulties in the FLO testing described above. One problem is knowing exactly the deflection plane associated with the twanging. An operator may think his deflection plane is vertical for instance, where in reality it may be several degrees off. Also, determining whether the shaft assembly is in FLO or wobbling can be difficult because of the relatively rapid rate at which the oscillation occurs. Sometimes means are employed to more easily see wobbling such as attaching a laser light to the oscillating shaft assembly and shining this light on a distant surface. These processes can be time consuming, require a relatively high degree of judgment by the operator, and have limited accuracy.
Desirable features of a golf shaft oscillation analyzer are an ability to determine the fundamental natural frequency and flat line oscillation planes of a golf shaft assembly.